Hi @wrongwright ,
It is all in this Lesson.
In the beginning the size is set with the first Cube to 4x4x8, as you say. Then the first floor is made and Separated into it’s own Object (and then Arrayed to about the top of the original Cube). That means the first Cube is now a ground floor followed by a gap for the first floor followed by the rest of the original Cube. That top part is the placeholder that is later being removed.
